Someone you are friends with on FaceBook, only on FaceBook, have never personally met, exchanged messages with, heard second-hand references to, or maybe not even seen more than a name and avatar of. Merely calling them "friend" almost insults the meaning of friendship, especially compared to your real friends or even passing aquaintances. In fact people call them "FaceBook Friend" or "Friend-on-FaceBook" knowing that God will punish them for using the word "friend" so cavalierly.

He's just a FBriend, I vaguely remember FBriending him when I was drunk and saw he had 56 FBriends in common with my best friend.